A radiographic phosphor panel contains a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-fight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conversion screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible light from irradiated phosphor crystals.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
Radiographic screens or panels have a support, a luminescent layer which includes the radiographic phosphor, and, generally, an overcoat layer to protect the phosphor layer. An overcoat layer is, desirably, substantially clear and transparent to the light emitted by the X-ray phosphor. To the extent that the overcoat absorbs any of the light emitted by the luminescent layer, the light output from the screen will be diminished, resulting in the need for increased X-ray exposure. An overcoat which is hazy, scattering the light used to stimulate the phosphor layer, can lead to a spreading-out of the stimulating light and a blurring of the image of fine objects, such as, cracks in bone or narrow blood vessels and an observed unsharpness and loss of diagnostic information.
An overcoat layer helps protect the phosphor layer underneath from mechanical damage due to scratches and abrasion, which would otherwise result in surface defects leading to artifacts in radiographs produced. In addition, an overcoat for all but the most moisture-resistant phosphors, must provide a barrier to the penetration of moisture, in the form of water vapor or liquid water, which would degrade the performance of the phosphor. Moisture penetration commonly has the effect of causing the panel to either have reduced light output, requiting the use of an increased X-ray dose to produce the same radiographic film density, or causing more localized dimmer areas as artifacts in resulting radiographs.
Degradation of final images due to phosphor panel discoloration is a particularly serious concern for radiation image storage panels, since unlike intensifying screens, storage panels are subject to cumulative degradative losses of both emitted light and stimulating radiation. There has not been agreement as to the mechanism of phosphor panel discoloration, but water and molecular iodine may both be involved.
What was noticed early was that intensifying panels subject to prolonged exposure to photographic film have tended to become discolored. U.S. Pat. Nos. 4,374,905 and 4,360,571 state that the discoloration is due to "volatile organic constituents escaping from the associated photographic film" (U.S. Pat. No. 4,374,905, column 1, lines 40-59 and U.S. Pat. No. 4,360,571, column 1, lines 46-64). Great Britain Patent Application No. GB 2 017 140 A states:
"[I]t has been discovered that screens containing lanthanum[or gadolinium]-oxy-halide phosphors tend to discolor rapidly when in use and in particular when held in contact with an X-ray film, . . . PA1 "In spite of intensive research into this discolouration defect the cause of it is not yet clearly known but it appears to be a complex reaction caused, in part at least, by the hydroscopic nature of the lanthanum-oxy-halide phosphors or gadolinium-oxy-halide phosphors, the nature of the binder and the presence of the X-ray film held in contact with the screen for a period of time. PA1 "Furthermore, under somewhat different conditions of use X-ray screens and in particular X-ray screens which contain lanthanum-oxyhalide or gadolinium-oxyhalide phosphors can lose speed due to a different defect which appears to involve only the phosphor. This is hydrolysis of the phosphor which is caused by water present in the phosphor layer due either to atmospheric moisture or aqueous cleaning fluid penetrating the protective layer of the screen. It is thought that quantities of halide or more surprisingly, the free halogen, released by hydrolysis may actually catalyse the discolouration of the binder or of compounds having migrated from the film." (page 1, lines 14-33)
Yellowing of a phosphor layer of a radiation image storage phosphor panel, in which the phosphor contains iodine, is described in European Patent Specification No. EP 0 234 385 B 1. The yellowing is ascribed to liberation of free iodine. An approach which has been taken to reduce storage panel yellowing is the incorporation of a stabilizer within one or more layers of the panel. European Patent Specification No. EP 0 234 385 B1 discloses as stabilizers a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and metal salts of particular organic acids.
Among materials which provide good moisture barrier properties are polymers having chlorine or fluorine containing monomer units, such as poly(vinylidene fluoride) and its copolymers. These materials, when used as X-ray panel overcoat layers, however, have the shortcoming of poor resistance to abrasion or scratches. In addition, many of these polymers form hazy films when solvent-cast. Overcoat layers have not proven particularly useful for preventing panel discoloration or yellowing associated with iodine.
U.S. Pat. No. 4,863,826 to Arakawa et al. describes a radiation image storage panel comprising a support, binder and stimulable phosphor particles characterized in that said stimulable phosphor particles are covered with a polymer material. The phosphor particles are in the form of a micro-capsule comprising a phosphor particle as as a core material and a polymer material as a shell. The polymer material is selected from the group consisting of polyester, nitrocellulose and polyakyl acrylate.
It would be desirable to provide improved radiographic panels and coated phosphors, in which yellowing or other environmentally-induced degradation is reduced.